The invention relates to the fields of dosimetry and calorimetry, in particular routine dosimetry using calorimetry for the routine monitoring and control of ionizing radiation processing procedures.
Radiation processing, the treatment of items with radiation, plays an important role in the production of many products. A radiation process is a method or procedure which uses radiation processing, such as radiation sterilization. Radiation may be used in a radiation process for the sterilization of materials, particularly for medical instruments and accessories, for the pasteurization of food products, and for material processing, such as for inducing or enhancing the polymerization of materials or the introduction of dopants or impurities into substantially pure materials. Electron-beam radiation is a common form of radiation used in radiation processes for sterilization, pasteurization, and for alteration of the properties of materials.
A radiation dosage is an amount of radiation absorbed by an irradiated item. Dosimetry is the measurement of an amount of radiation.
Measurement, by means of a dosimetry system, and reporting of the amount of radiation absorbed by items to be sterilized or otherwise processed by radiation is an important quality control measure. Reporting requirements in highly regulated industries, such as those typically using radiation processing, are stringent and often require much effort. Calibration of the dosimetry system, comprising comparison of the dosimetry system measured values with those of national standard radiation sources with different control settings, is typically performed before use of a radiation source, and optionally at other times thereafter, to determine whether the dosimetry system is providing accurate radiation dosage measurements. The calibrated dosimetry system is used, if necessary, to adjust the radiation source so that it performs as desired. Quality control measurements may be routine measurements taken infrequently or frequently, and often at regular intervals, to monitor the performance of the radiation processing on an on-going basis. Such quality control measurements of radiation delivery during radiation processing are termed routine dosimetry measurements, and differ from calibration measurements in that calibration measurements are used to assure the accuracy of the dosimetry system, and to verify that dose measurements obtained are valid, while routine dosimetry measurements track the performance of a radiation process during use.
Two general categories of dosimeters exist, reference standard dosimeters and routine dosimeters. The objective of the reference standard dosimeters is to provide a link between national standard dosimetry calibration laboratories and production radiation processing facilities. The key criteria is control and accuracy of measured dose. Practicality is thus not an issue for reference standard dosimeters since their use during calibration of routine dosimeters is infrequent. Routine dosimeters, on the other hand, are used for regular quality control within a radiation processing plant. Practicality is of utmost importance, combined with reasonable accuracy.
The amount of radiation delivered by an electron beam may be measured in a number of ways. For example, the current induced by the passage of radiation past capacitor plates was reported as a measure of the radiation beam dosage by Taumann, U.S. Pat. No. 4,427,890, while the current produced in a coaxial sensor placed in an electron beam was reported to be proportional to an electron beam current intercepted by the sensor, and so was said to be useful as a measure of the beam dosage (Fiorito et al., U.S. Pat. No. 4,629,975). The current collected by a beam stop was said by Lawrence et al., U.S. Pat. No. 5,661,305, to be a useful measure of the absorbed dose in a product irradiated by an electron beam. All patents cited herein, both supra and infra, are hereby incorporated by reference in their entirety.
A listing of and a discussion of the advantages and disadvantages of various dosimetry methods may be found in Annex C of xe2x80x9cDosimeters, dosimetry and associated equipmentxe2x80x9d ANSI/AAMI/ISO 11137-1995 (1995) (referred to herein as xe2x80x9cISO 11137xe2x80x9d). For example, calorimetry is listed therein for use as a reference standard dosimeter, while several spectrophotometric methods are listed as examples of routine dosimeters.
One method of detecting and measuring radiation is to measure the optical density of radiation-sensitive films (also known as radiochromic films) exposed to radiation. Commonly, calibration is performed by irradiation of unexposed radiation-sensitive film routine dosimeters together with controlled reference standard dosimeters from national standards laboratories. The results of such film dosimetry are determined by the amount of exposure of the film following irradiation, such as by measuring the optical density of the developed film. Results of the reference standard dosimeters are then correlated with the results of the routine film dosimeters to form calibration curve for the film dosimeters. Film dosimetry is performed for routine dosimetry to confirm that the appropriate amount of radiation is being delivered to the items to be irradiated, and to correct the exposure if it is found to be inaccurate. However, there is a delay in obtaining film dosimetry readings while the film is developed, and film dosimetry is subject to problems of reliability and consistency due to variation between films, variability of film placement on or within the items being irradiated, variability in the time between irradiation and film development, and the effect of temperature, humidity and ultraviolet light levels on the exposure characteristics of the film (see, for example, Table C5, ISO 11137).
The absorption of radiation heats an object that has been irradiated. Calorimetry, which measures heat, may be used as a method of dosimetry by measuring a temperature change in an irradiated item and correlating the radiation dosage absorbed with the temperature change. However, in order to be accurate, these measurements must be made without allowing any significant loss of heat. Radiation processing with gamma radiation, for example, may take several hours. Electron beam sources typically require a shielding maze and multiple passes within the maze, requiring an hour or more for radiation processing. Significant amounts of heat transfer might occur during this time, making calorimetry measurements difficult and inaccurate under these circumstances. Attempts to calibrate an electron beam source radiation by placing a calorimeter on an arm that swings into the path of the electron beam require that radiation processing of other items be stopped, and the system reconfigured for normal operation before radiation processing is able to begin again. However, such a system cannot be used for routine dosimetry measurements because the calorimeter interrupts the normal functioning of the system.
The variability in the measuring tools presently used in routine dosimetry hinders routine measurement of the performance and reliability of radiation sources. However, radiation processing, such as for sterilization, is a heavily audited and critical process in the production of medical implements and medical instrumentation, in food processing, and in the processing of many materials. The ability to minimize quality control issues related to routine dosimetry would improve present methods and significantly reduce the risk of non-compliance with the strict regulations that are typically applied to radiation processing.
Accordingly, what is required are systems and methods for radiation processing and for routine dosimetry that are not affected by sensor location, variability in sensing elements, and other such problems, and are capable of being used without interference with the normal operation of radiation processing methods.
The present invention is directed to a method and system for determining a radiation dose for the quality control of a radiation process, and particularly to a method and system for routine dosimetry for the sterilization of medical products.
A system embodying features of the invention, such as a system for routine dosimetry, includes a calorimeter, particularly a thermistor calorimeter, a radiation source, and a calorimetry control system. Such systems are particularly suitable for use in sterilizing medical products.
The calorimeter should have a validated resistance-temperature calibration relationship and a validated temperature-dosage relationship. Irradiation of the calorimeter heats the calorimeter. The temperature of a calorimeter after irradiation is preferably measured before significant heat loss has occurred. In routine dosimetry, calorimeter temperature is measured as soon as possible after irradiation so that only very small amounts of heat are lost. Preferably, the loss of heat from the calorimeter is a linear function of time after irradiation. In embodiments of the system, the calorimeter is configured to reduce or prevent heat loss, such as by insulation or by maintaining the external temperature near to the temperature of the heated calorimeter.
The radiation source may be a controlled radiation source, and is preferably a high dose-rate radiation source. The high dose-rate radiation source may be an electron radiation source. Radiation doses provided by the radiation sources of the systems of the invention may be between about 0.1 kGy to about 100 kGy. In embodiments, the radiation dose is between about 2 kGy to about 70 kGy, and in still further embodiments, the radiation dose is between about 3 kGy to about 40 kGy.
The calorimetry control system may include a calorimeter controller, which may monitor and/or direct the performance of the system and its components. The calorimeter controller should be an automatic calorimeter controller, and preferably should be a computer-controlled automatic calorimeter controller. In routine dosimetry control the interval between routine dosimetry measurements is constant, unless the target radiation dose has changed, in which case a routine dosimetry measurement is taken regardless of the amount of time that has passed since the previous routine dosimetry measurement. Typically, the interval between dosage determinations is less than about an hour, preferably less than about a half hour.
The systems may further include a conveyor system for effecting relative motion between a radiation source and items and calorimeters to be irradiated. The conveyor should be effective to move the calorimeter through the path of radiation from the radiation source, preferably within a short time. The conveyor may move the calorimeter along a short, closed-loop route so as to return the calorimeter to and ending position within a short time. In embodiments, the ending position is or is near to the starting position.
In addition, the systems of the invention may further include a robotic arm having a resistance measuring device able to temporarily contact and obtain a resistance measurement from the thermistor calorimeter.
The present invention further provides methods for routinely determining and reporting a radiation dose for quality control of a radiation process, which is particularly suited to routine dosimetry for the sterilization of medical products. Routine dosimetry methods embodying features of the invention include measuring an initial calorimeter temperature, irradiating a calorimeter, and measuring a subsequent calorimeter temperature.
The calorimeter temperature is measured before and after irradiation of the calorimeter by the radiation source, and the radiation dose received by the calorimeter is determined using the calculated temperature difference between the initial temperature and subsequent temperature measurements of the calorimeter, and using the resistance-temperature and temperature-dose calibration relationships in conjunction with the temperature differential. Preferably, the irradiation of the calorimeter is performed in the same manner as the irradiation of items, such as medical products receiving radiation sterilization, is performed. The dose determination procedure is preferably repeated at an interval, or after a specified number of items has been processed, or by other criteria, determined by the calorimeter controller. The determined dosage may then be reported.
The calorimeter controller may be used to determine and/or control the interval between routine dosimetry measurements. For example, the calorimeter controller may be used to determine whether the dose from the radiation source has been changed, and to maintain the interval between routine dosimetry measurements constant when the radiation dose has not changed. The calorimeter controller may also be programmed to prompt or initiate a calorimeter measurement whenever the radiation dose from the radiation source has changed. In addition, the calorimeter controller may be employed to determine whether the calorimeter has validated resistance-temperature and temperature-dosage relationships, i.e., is a validated calorimeter, may be programmed to accept data only from validated calorimeters, and may manage the printing of a process report.
Thus, a method embodying features of the invention comprises: providing a calorimeter control system and a calorimeter; measuring an initial calorimeter temperature; irradiating the calorimeter with a dose of radiation from a radiation source; measuring a subsequent calorimeter temperature before significant heat loss has occurred; determining the radiation dose using a calculated temperature difference between the initial temperature and subsequent temperature measurements, and using the resistance-temperature and the temperature-dosage calibration relationships; repeating these procedures at an interval determined by the calorimeter controller; and reporting the radiation dose. In embodiments, the calorimeter is a thermistor calorimeter, preferably a validated thermistor calorimeter having a validated resistance-temperature calibration relationship and a validated temperature-dosage calibration relationship.
Preferably, the calorimeter measurements taken subsequent to irradiation are taken as soon as possible after irradiation so that only small quantities of heat are lost from the calorimeter, or so that the heat loss will be a nearly linear function of time after irradiation. To minimize such heat loss, and so to measure the calorimeter temperature before significant heat loss has occurred, the time the calorimeter temperature is taken after irradiation should be a short time, such as less than about an hour, preferably less than about 30 minutes, and more preferably less than about 15 minutes. Other methods may also be used for minimizing heat loss and insuring that calorimeter measurements may be made before significant heat loss has occurred, such as providing insulation, raising environmental temperature, or other methods.
In embodiments of the routine dosimetry method, the irradiating procedure further comprises movement of a calorimeter by a conveyor along a route. Such a route may be a short, closed-loop route. The calorimeter may be conveyed along a route at a rate effective to return it to its starting position within a short time, or to an ending position which may be near to the starting position, within a short time. Such a short time may be less then about 30 minutes or, preferably, may be less than about 15 minutes. In embodiments of the methods, calorimeter temperature measurements are taken at locations determined by the conveyor route, typically at locations near to the starting position and/or ending position along the conveyor. In embodiments of the method, the time required to convey a calorimeter along a route between starting and ending positions is substantially the same as the time interval between initial and subsequent calorimeter measurements. Such a time interval is preferably a short time.
In one embodiment, the radiation source is a high dose-rate radiation source, such as an electron radiation source. The radiation dosage output capability of the radiation source should be between about 0.1 kGy to about 100 kGy, preferably between about 2 kGy to about 70 kGy. For sterilizing medical products such as guidewires, catheters, and the like for vascular procedures, suitable dosage rates may range from about 3 kGy to about 40 kGy.
Determining a radiation dose includes contacting the thermistor calorimeter with a measuring device and at least temporarily moving the temperature measuring device while in contact with the thermistor calorimeter to obtain a calorimetry measurement from the calorimeter. The temperature measurement preferably is a resistance measurement.
The invention also provides a routine dosimetry control method for determining at intervals and reporting an acceptable radiation dose in a radiation process comprising a calorimeter that has a maximum lifetime dose. Such embodiments of the routine dosimetry control methods comprise determining whether a calorimeter is valid and has received less than a maximum lifetime radiation dose. The calorimeter is determined to be a validated calorimeter if it is a valid calorimeter and has received less than a maximum lifetime radiation dose.
The invention yet further provides a method for radiation processing of an item, such as a medical product, comprising processing a calorimeter in a radiation process according to a routine dosimetry method of the invention, and processing an item in the radiation process. Preferably, the calorimeter and the processed items are irradiated in the same manner, preferably by the same radiation source.
In addition to the particular embodiments listed above, it will be understood that other combinations and arrangements of the components and procedures of the systems and methods described may be used in the practice of the invention. All such combinations and arrangements are within the scope of the present invention.
The routine dosimetry systems and methods of the present invention provide benefits that include reduced labor requirements, reduced environmental requirements, and increased consistency and quality of dosimetry results for both processing dosimeters and investigating dosimetry issues. Other dosimetry methods commonly used at present, such as film or other spectrophotometric methods, have disadvantages such as requiring the calibration of optical equipment and the maintenance of special environments.
Practice of the methods of the present invention are effective to reduce the labor needed to perform a radiation process; for example, an automatic calorimeter controller eliminates the need for a person to read or process dosimeters, improves reliability and repeatability of the processes, thereby reducing stoppages and reducing wastage. Automatic control of a radiation processing system allows for the automatic collection and recording of data for process reports, reducing the costs of compliance with regulatory requirements. Thus, the radiation processing and routine dosimetry as performed in the practice the present invention can be accomplished with fewer resources, with far superior quality and with far less compliance risk than prior methods.
These and other advantages of the invention will become more apparent from the following detailed description of the invention and the accompanying exemplary drawings.